There are many types of coin operated devices, and almost as many ways to attempt to cheat them. Most commonly, slugs or other cheaply manufactured "coins" are used to mimic the tested characteristics of acceptable coins. The problem can be particularly acute in casinos where coin operated gaming devices, such as slot machines, are configured to operate on relatively expensive tokens which are manufactured by the casino, not minted by the government. The metal content and other characteristics of the tokens can vary over time, or from casino to casino, and coin testers must be configured to accept the relatively wide range of valid tokens, while rejecting counterfeits. Since the manufacturing cost of the coin or an imitation is substantially less than its assigned casino value, the manufacture distribution or use of counterfeit coins can be very lucrative, and it is not always a simple task to distinguish between manufactured tokens intended to be acceptable and those which are fraudulent. The foregoing case is given as simply one example of the difficulty of distinguishing between acceptable coins and unacceptable counterfeits.
Early mechanical coin testers which functioned on coin size or weight were easily defrauded by slugs intended to mimic the size and weight of the originals. A particularly successful modern coin tester is the electronic device disclosed in Nicholson et al. U.S. Pat. No. 4,469,213. That system relies on comparing the magnetic properties of a sample coin to those of a deposited coin; such system has significant ability to distinguish between acceptable genuine coins and unacceptable counterfeits. However, a number of instances, one of which was in the slot machine casino environment, have rendered that system less than completely effective. That is particularly true in the casino type case where a number of casinos manufacture coins of a given denomination which can be used interchangeably in the machines of the various casinos. Those tokens being relatively inexpensively manufactured tend to wear. In addition, the tokens tend to vary in metal content, in one example tokens comprising a nickel silver alloy varying from 10% to 25% in nickel content. It has been found necessary to "detune" the circuitry of the aforementioned coin tester in order to provide a sufficiently broad response to accept the rather wide range of acceptable coin characteristics. When the system is detuned, it loses a certain amount of its ability to discriminate between acceptable and counterfeit coins.